1. Field of the Invention
This invention relates to a magnetic recording medium installed in a variety of magnetic recording devices, such as an external storage of a computer, and a method of manufacturing the same. More particularly, this invention relates to an “anisotropic” magnetic recording medium that is fabricated using a substrate made of a glass material, and a method of manufacturing such a magnetic recording medium in an inexpensive and simple manner.
2. Description of the Related Art
Generally, a magnetic recording medium using a substrate made of an aluminum material is textured to form circular grooves (called “texture”) on a surface of the substrate. A seed layer, an under layer, a magnetic layer, and a protective layer are sequentially laminated on the textured substrate. The texturing is intended to prevent the magnetic recording medium from coming into contact with a magnetic head when the magnetic head flies and seeks over the magnetic recording medium, and to raise a recording density by orienting a direction of an in-plane magnetization to a circumferential direction to change the recording medium into a so-called “anisotropic” medium. Usually, the circular grooves are formed on the substrate made with the aluminum material for the magnetic recording medium by plating a blank of aluminum with NiP and texturing the surface of the substrate. A distortion and the circular grooves are caused by a texturing result in a difference in remanent magnetization between the circumferential direction and a radial direction when the under layer and the magnetic layer are formed.
On the other hand, magnetic recording media using substrates made of glass materials are usually so-called “isotropic media” which are not textured for the following reasons: 1) the materials have a high hardness and reliability; 2) the materials have a low thermal expansivity; and 3) even if the substrate is textured as the substrates made of aluminum materials, there is no difference between the remanent magnetization in the circumferential direction and the remanent magnetization in the radial direction. In this specification, the “isotropic medium” means a medium including a ratio of the remanent magnetization in the circumferential direction to the remanent magnetization in the radial direction being substantially 1 (e.g. 0.95 to 1.05), and the “anisotropic medium” means a medium having a ratio of the remanent magnetization in the circumferential direction to the remanent magnetization in the radial direction equal to or greater than 1.05.
With an increase in the recording density of magnetic recording media in recent years, a one-bit recording region (bit size) which is a minimum unit of data written in the recording medium has become smaller and smaller. As described above, the conventional magnetic recording media using glass substrates are mainly isotropic media, but with a decrease in bit size, a difference between the isotropic media and the anisotropic media has become conspicuous because the isotropic media have exhibited a lower resolution (the percentage of an MF output with respect to an LF output) and a deteriorated S/N ratio (SNR: signal-to-noise ratio). Furthermore, because a necessity exists of increasing the remanent magnetization in the circumferential direction (Mrt-Cir) so that the isotropic media can achieve the same characteristics as the anisotropic media, an increase occurs in the thickness of magnetic layers, which results in an increase in media noise generated by the magnetic layers.
To prevent the increase in the media noise in the isotropic media, pure Ar for sputtering has been used for sputtering the base layer to the magnetic layer, and on the other hand, U.S. Pat. No. 5,866,227, incorporated herein by reference, discloses forming a NiP or Ta seed layer in mixed gas of Ar and oxygen by reactive sputtering to improve a crystal orientation of the magnetic layer to lower the noise. Further, U.S. Pat. No. 5,879,783, incorporated herein by reference, discloses exposing a surface of an NiP seed layer to mixed gas of Ar and oxygen and then increasing a surface roughness of the NiP seed layer to, thus, reduce a size of crystal particles in the magnetic layer. These patents, however, as not disclose that the substrate and the surface of the seed layer have in-plane anisotropy in terms of shape. Therefore, magnetic recording media disclosed in these publications have substantially the same value of in-plane remanent magnetization in the circumferential direction and the radial direction of the substrate and have the similar isotropic magnetic characteristics.